I'm not so sure performance would be overall retrograded so long as there's enhanced parallelism on write-backs with QLC NAND. But that's going to be entirely dependent on free space / over provisioning slack and how manufactures implement this new technology. So at best, there's an improvement. At worst, it is in fact degraded. I'm thinking that's going to vary a lot between vendors.
As for endurance, agreed. It will be worse.
So long as the cost per GB goes down in accordance with the QLC, it might actually make sense to keep the OS drive with TLC, and relegate QLC for larger 1TB+ data drives where they're mainly read-only archive/media storage. But then again, if I'm going to use a separate secondary data volume, I'd probably go with a 6+ TB HDD with additional HDDs used for backing up all that data too. Of course, it's entirely situational to your needs.